1. Field of the Invention
The present invention relates to a camera shake compensating optical system for compensating for blur of an image taken when a taking optical system inclines due to camera shake caused at the time of photographing performed while the camera is being held by the hands.
2. Description of the Prior Art
Conventionally, failures in photographing mostly resulted from camera shake and being out of focus. However, since most of the recent cameras are provided with an automatic focusing mechanism and the automatic focusing mechanism has improved focusing accuracy, the problem of the photographing failure due to being out of focus has been nearly solved.
On the other hand, the lens system normally incorporated in the camera has shifted from a fixed focal length lens system to a zoom lens system, and with the shift, the magnification and the longest focal length have been increased. Consequently, camera shake very frequently occurs.
As a result, presently, it is no exaggeration to say that the failure in photographing is caused by camera shake. For this reason, an optical system for compensating for image blur due to camera shake is indispensable.
Conventionally, a variable vertical angle prism has been known as an optical system for compensating for the image blur which optical system is added to a taking optical system which is a main optical system (Japanese Laid-open Patent Application No. S50-112054). As examples of the variable vertical angle prism, the following two are well known: the one (first prior art) in which a liquid L is enclosed between two glass plates GP1 and GP2 as shown in FIG. 1(a) and the glass plates GP2 is inclined by moving one end thereof (in the direction of arrow m1) as shown in FIG. 1(b); and the one (second prior art) in which a plano-concave lens GL1 and a plano-convex lens GL2 are attached to each other at the spherical surfaces as shown in FIG. 2(a) and one of the lenses is decentered along the attached spherical surfaces (in the direction of arrow m2) as shown in FIG. 2(b).
U.S. Pat. No. 2,959,088 proposes an afocal lens system including a concave lens and a convex lens in which the convex lens is rotated about the focus position of the convex lens. In this system, an angle of camera shake (an angle at which a light beam is to be bent) and an angle of compensation (an angle at which the convex lens is rotated) are of the same magnitude but in the opposite direction. Since the optical axis of the convex lens is always horizontal by means of a weight, the optical axis of the convex lens is not moved by any position changes, so that camera shake is compensated for. With such an arrangement, this lens system requires no detecting system for detecting camera shake amount and no driving system.
As another optical system for compensating for camera shake, Japanese Laid-open Patent Application No. H2-93620 proposes an optical system in which a part of a taking optical system is decentered.
Japanese Laid-open Patent Applications Nos. H1-116619, H1-189621, H1-191112 and H1-191113 disclose numerical embodiments of three-unit zoom lens systems (concave, convex and concave from the object side) in which camera shake is compensated for by independently decentering each lens unit.
Problems encountered by the variable vertical angle prism will be described with reference to FIGS. 3 and 4.
FIG. 3 shows a variable vertical angle prism (third prior art) including a plano-concave lens G1 and a plano-convex lens G2. Surfaces R1 and R4 are nearly plane surfaces. Surfaces R2 and R3 are spherical surfaces with a radius of curvature of approximately 40 mm. The absolute values f1 of the focal lengths of lenses G1 and G2 are both approximately 57 mm. The lenses G1 and G2 constitute an afocal system. In the figure, the numerals -20, -15, -10, 0, +10, +15, +20 on the left side of the variable vertical angle prism represent image heights when the focal length of the taking optical system is 35 mm. Optical paths of incident light beams corresponding to the image heights are also shown.
FIG. 4 shows a condition in which the lens G2 is decentered along the spherical surface R3. Since the center of decentering is the center of curvature of the spherical surface R3, even though the spherical surface R3 moves by the decentering of the lens G2, optically, the position of the spherical surface R3 does not change. On the other hand, the plane surface R4 inclines as the lens G2 is decentered. As a result, a transmission deflection angle is obtained similarly to the previously-mentioned second prior art (FIG. 2). The dotted lines in the figure represent the position of the variable vertical angle prism and optical paths (FIG. 3) before the decentering.
In this third prior art, in order to correct an axial light beam by 1.degree., the lens G2 is rotated by approximately 1.4.degree. along the spherical surface R3. With this arrangement, a transparent optical device is obtained which is equivalent to the case where a wedge-shaped prism with a vertical angle of approximately 1.4.degree..
However, as shown in FIG. 4, when the taking optical system is of a wide angle (focal length: 35 mm) as described above, a light beam off an optical axis AX of the taking optical system (hereinafter referred to as off-axial light beam) is bent by as much as 1.27.degree. at the most with respect to a compensation angle of 1.01.degree. of a light beam on the optical axis AX (hereinafter referred to as axial light beam). This means that when blur of an axial image due to camera shake is compensated for, an off-axial image is over-compensated for, which adversely causes image blur. The amount of the blur caused by the over compensation is as much as 106 .mu.m in the case of compensation in a taking optical system with a focal length of 35 mm and an image height of 15 mm.
From the above, it is found that in a case where the film size is 35 mm, it is when the focal length is 100 mm or larger and the maximum taking angle of view is up to approximately 12.degree. that both axial and off-axial images of the taking optical system can excellently be compensated for by means of the variable vertical angle prism.
On the other hand, the system of the above-mentioned U.S. Pat. No. 2,959,088 presents the following problems. First, since the center of rotation of the convex lens is away from the convex lens, in order to support the convex lens by means of a gimbal, the size of the system should be increased. Secondly, similarly to the case of the variable vertical angle prism, the off-axial light beam is over-compensated for when the taking optical system has a short focal length.
FIG. 5 shows a system (fourth prior art) where in a system similar to that of the third prior art, the lens G2 is rotated about its focus position similarly to the one proposed by U.S. Pat. No. 2,959,088. In the system of the fourth prior art, in order to compensate for the axial light beam by 1.degree., the lens G2 is rotated about its focus position by approximately 1.degree.. However, as shown in the figure, the off-axial light beam is over-compensated for by 20% with respect to the compensation of approximately 1.degree. of the axial light beam.
Japanese Laid-open Patent Applications Nos. H2-238429 to H2-238431, H2-239220, H2-239221, and H2-240622 to H2-240624 also propose optical systems with the same object as that of U.S. Pat. No. 2,959,088. In these optical systems, in order to decrease the demerit of U.S. Pat. No. 2,959,088 that the size is large, the distance from the convex lens to the center of rotation is made shorter than the focal length of the convex lens by changing the power distribution. However, the problem that the size is large has not completely been solved.
Moreover, the over compensation of the off-axial light beam is larger than that of U.S. Pat. No. 2,959,088. Japanese Laid-open Patent Application No. H2-238429 teaches in its specification that images off the optical axis do not move either. However, this is with respect to the paraxial ray tracing and the over compensation results from the fact that the paraxial theory does not apply to the wide angle. In this method, in order to solve the problem of the over compensation of the off-axial light beam, it is necessary to increase the number of lens elements in both the concave and the convex lens units. If the number of lens elements increases, the cost will inevitably increase.
In the system of the above-mentioned Japanese Laid-open Patent Application No. H2-240624, since the concave lens unit and the convex lens unit are away from each other and the rear surface of the convex lens is inclined by the decentering of the convex lens, it is difficult to make excellent compensation at a wide angle lens. Moreover, the number of lens elements of the camera shake compensating optical system is as large as four and the diameter thereof is large, the size is large and the cost is high.
In the above-mentioned system where a part of the taking optical system is decentered, it is necessary to decenter a lens in accordance with each taking optical system. Particularly in the zoom lens system, it is extremely difficult to compensate for camera shake while maintaining an excellent image at any focal length during zooming.
In the system of the above-mentioned Japanese Laid-open Patent Application No. H1-116619 where the first lens unit is decentered, the over compensation amount of the off-axial light beam is considerably large at the shortest focal length condition and the aberration deterioration due to the decentering is remarkable at the longest focal length condition. In the system of Japanese Laid-open Patent Application No. H1-116619 where the second and third lens units are decentered, the displacement amount of the decentered lens unit for compensating for camera shake is changed when the focal length of the zoom lens system is changed. As a result, in order to calculate the displacement amount of the decentered lens unit, highly accurate information is required on the camera shake angle and the focal length of the zoom lens system. Since in consideration of the non-uniformity of each part, a considerably complicated adjustment is required, the system of this prior art is unsuitable for mass production.